The present invention is directed to integrated circuits and, more particularly, to an input buffer with selectable hysteresis and speed.
An input buffer is used in integrated circuits (ICs) to provide an interface between an external source of an input signal and an internal module. The source of the input signal may be connected to an input pad or an input/output (I/O) pad. In the case where the buffer is connected to an I/O pad, it is commonly desirable for the buffer to be enabled when the I/O pad is used as an input but disabled when the pad is used as an output.
Input signals from an external source are often subject to noise, especially caused by interference induced in the connection between the external source and the input pad. Typically, an input buffer is designed with transfer characteristics that exhibit hysteresis. Hysteresis of the transfer characteristics of an input buffer makes a change in the output signal of the buffer dependent on the history of past values of the input signal. Accordingly, if a first change of the input signal initially produces assertion of the output signal, a second and opposite change of the input signal in order to de-assert the output signal will have to be of bigger magnitude. The hysteresis has the effect of filtering noise of small amplitude compared to the input signal.
The transfer characteristics that confer hysteresis tend to conflict with the characteristics that confer fast operating speed. Also, higher hysteresis may be desirable in a slower speed buffer. For a given input pad or I/O pad, it may be desirable to select by a control signal whether the transfer characteristics have higher hysteresis and slower speed, or higher speed and lower hysteresis. It is possible to achieve the selection of transfer characteristics by providing two input buffers that can be selected alternatively by the control signal enabling one and disabling the other. However, such an approach tends to consume area and increase cost.
It would be advantageous to have an input buffer having alternative hysteresis and operating speed characteristics that can be selected by a control signal.